We have continued our studies on the neurotransmitter receptors in the gastrointestinal tract by using RNA probes to detect mRNA. We have determined that the H2 receptor mRNA is present in immune cells of the lamina propria, and in the epithelium of the stomach and duodenum. We found that only about one-third of the parietal these cells bear the H2 receptor, while all mucus-containing cells in both the stomach and the duodenum synthesize the mRNA for the receptor. We have done similar double-labeling studies for the muscarinic, dopaminergic, and gastrin receptors, and are currently evaluating those results. We will soon have the result of our experiments to detect changes in the distribution of the above mRNAs in different stresses and in ulcerated stomachs. Among the gastrointestinal neuropeptides, we have performed several studies on CGRP, SP, and VIP.These peptides are present in both neuronal and cellular elements in the GI tract. CGRP-positive cells are often seen in close association with peptidergic peripheral sensory nerves, mainly in the duodenum. On the other hand, an intimate relationship between leukocytes and parietal cells was observed in the gastric glands suggesting their mediatory role in gastric acid secretion. Eosinophilic leukocytes have readily hybridized with two 35S-labeled oligonucleotide probes with nonoverlapping sequences of rat CGRP gene. This phenomenon may reflect an interactive role of peptidergic afferent nerve fibers and immune cells in the protection of gastric mucosa against ulcers induced by chemical factors or stress. We also begun to study the role of substance P (SP) in the pathogenesis of a toxin-mediated diarrhea and mucosal inflammation. Immune cells in the lamina propria contain SP and make its receptor. We have found a huge increase in SP receptor mRNA in both mucosal and lamina propria cells in the toxin-treated ileum compared to the control. At present, we are doing experiments to see how administration of an SP antagonist prior to toxin treatment might affect the invasion of inflammatory cells into the ileum. Using RNAse protection assay, we have demonstrated that there is a high amount of SP receptor mRNA in the small intestine.